Diffusion coating



l 20, 1970 H. D. FLICKER 3,535,145

DIFFUSION COATING Filed May 2, 1967 INVENTOR 3,535,146 DIFFUSION COATINGHoward D. Flicker, North Miami, Fla., assignor to Aircraft Plating Inc.,Miami, Fla., a corporation of Florida Filed May 2, 1967, Ser. No.635,476 Int. Cl. B44d 1/14 US. (11. 11771 20 Claims ABSTRACT OF THEDISCLOSURE There is provided a surfaced alloyed metal product and amethod for production thereof. The metal part to be surface alloyed iscoated with a decomposable compound, coated with an alloying material,and heated to an elevated temperature above the decompositiontemperature of the decomposable compound in a dry gaseous atmosphere,especially in a hydrogen atmosphere. The decomposable compound mustcontain at least one element which has an atomic volume greater than theatomic volumes of the elements of the metal part to be surface alloyedand the alloying materials. Preferably the decomposable compoundcontains an element which has an atomic volume at least 1.25 times theatomic volumes of the metal part and alloying material.

This invention relates to a novel surfaced alloyed metal article orsubstrate and to a method for production thereof.

It is known in the art to form surfaced alloyed metal articles bytechniques generally referred to as diffusion alloying or diffusioncoating. Heretofore, such articles were produced by a number of methodssuch as: a vacuum chamber or sputtering technique to coat the metalarticle with a second metal composite and heating to cause diffusion andalloying; plating the metal article with a second metal and thereafterheating to cause diffusion and alloying; packing a metal powder aroundthe metal article and heating to cause dicusion and alloying.

While all of the above-mentioned techniques provided useful products,the articles so produced suffer from common disadvantages. When suchsurface alloyed products are subjected in service at high temperatures,such as the temperature commonly encountered in jet engines, thealloying metals or compound contained in the coating begin to furtherdiffuse into the metal part whereby the composition of the alloyedsurface begins to radically change. If exposed for extended periods oftime or for short periods of time at very high temperatures, thisdiffusion will diminish the amount and proportions of the alloyingmetals of the surface of the part to a point where' the usefulproperties of the alloyed surface no longer exists and the part becomesunserviceable or, even worse, fails in use. Furthermore, the diffusedcoating thicknesses are difficult, if not impossible to control. Eachprocess has limiting factors, for instance, because of the high vaporpressure, chromium cannot readily be diffused in vacuum. Aluminum cannotbe diffused in atmospheres containing nitrogen, etc. Thus, two of theelements which are among the best for oxidation resistance can only bediffused with limited techniques.

It is, therefore, an object of this invention to provide a surfacecoated or alloyed metal article which will be substantially resistant tofurther diffusion at elevated temperatures. Another object is to providea process for producing a surface coated or alloyed part which is simplein operation, inexpensive and will produce a surface coated alloyed partthat is substantially resistant to further diffusion at elevatedtemperatures. Oher objects will be apparent from the followingdisclosure and claims.

3,535,146 Patented Oct. 20, 1970 Briefly stated, the above objectivesare accomplished by applying a coating on the metal substrate of atleast one decomposable compound, applying a coating over the saiddecomposable compound of at least one alloying material, and subjectingthe so coated metal substrate to an elevated temperature above thedecomposition temperature of the said decomposable compound in a dryatmosphere, wherein the said decomposable compound contains at least oneelement which has an atomic volume greater than the atomic volumes ofthe elements of the said metal substrate and the said alloyingmaterials.

The decomposable compounds must contain elements which have a relativelylarge atomic volumes as compared to the atomic volumes of the substrateand alloying materials and preferably the atomic volume should be atleast 1.25 times greater, as explained in greater detail hereafter.

The decomposable compounds must be chosen so that the decompositiontemperature thereof is lower than the solidus (melting-freezing)temperature of the substrate metal, and it is preferable that thedecomposition temperature should be at least F. lower than the solidustemperature of the substrate material. However to avoid decompositionprematurely, the decomposition temperature should not be more than 1100F. lower than the solidus temperature of the substrate. However, thedecomposition temperature should be at least 350 F.

The decomposable compounds are conveniently applied to the substratewith a lacquer made of any of the known organic film-forming materialswhich will be substantially burned away (non-carbonizing) at theelevated temperature utilized in the alloying step. While a host of suchfilm-forming materials is known to the art, typical examples includepolyvinyl chlorides, acetates and alcohols, polyesters, epoxies,nitrocellulose, polyolefins, natural and synthetic rubbers such asbutadienestyrene, butyl, and neoprene, drying oils such as linseed,perilla and tung oils, and polyurethanes. The lacquer performs thefunction of holding the decomposable compounds in place while thetemperature of the substrate is being raised. However, where thedecomposable compounds decompose at relatively low temperatures, caremust be exercised in choosing the particular lacquer since it isnecessary that the lacquer be substantially burned away(non-carbonizing). Of course, a suitable solvent for the film-formingmaterial will be necessary to prepare the lacquer and the amount ofsolvent will vary with the particular film-former and the viscosity ofthe solution desired. It has been found that a particularly good lacquermay be prepared by dissolving pyroxylin in Cellosolve (ethylene glycolmonoethyl ether, manufactured by the Union Carbide Corp), in a ratio ofabout 3:100 to 20:100 by weight.

While the alloying materials may be in any soluble form with regards toa particular solvent, it is most convenient to use water solublecompounds such as salts or oxides of the alloying elements.Advantageously, small amounts of matrix forming agents are added to thesolution such as, ,1 to 2% of a soluble alginate. Preferably, thesolution of the alloying materials is made by dissolving the materialsin an aqueous solution of a silicate or borate that has been neutralizedwith a mineml acid such as hydrochloric, sulphuric or nitric acid.Suitable silicates or borates include the organic esters of acids suchas ethyl and methyl silicates and borates, ammonium silicates, and thealkali metal silicates and borates such as sodium silicate The silicatesand borates are especially valuable since these not only form a matrixfor holding the alloying metals but also have the property of adding asmall silicon and/or boron component to the diffusion coating. Siliconand boron generally possess the property of creating an eutectic withother metallic elements. The

amount of silicate or borate may be up to dependent on the amount ofsilicon or boron desired in the diffused coat.

While it is preferable to use a saturated solution of the alloyingmaterials, this is not necessary, and any proportion of alloyingmaterials may be used. The particular ratio of the various alloyingmaterials depends on the ultimate proportion of alloying elementsdesired in the surface alloyed substrate. According to the invention, awide variety of materials may be surface alloyed into a metal substrateand include chromium, aluminum, iron, zirconium, hafnium, silicon,nickel, titanium, tungsten, molybdenum, yttrium, columbium, cobalt,palladium, selenium, gold, vanadium and manganese The abovementionedmaterials are not all inclusive and are intended merely to illustratethe Wide application of the present invention.

The temperatures used for drying the lacquer containing the decomposablecompound and the coating containing the solution of alloying materialswill depend on the particular lacquer and coating composition. Generallyspeaking, moderate temperatures are quite sufficient and fall within therange of room temperature to 500 F. The time of drying will depend onthe temperature, speed of air currents over the coating, humidity andthickness of substrate but will generally be between 2 and minutes. Ofcourse, other temperatures and times may be used if desired. Forexample, when pyroxylin dissolved in Cellosolve is used as the lacquer,140 F. is a satisfactory temperature. And when the solution of thealloying metals contains aqueous sodium silicate, 200 to 400 F. is asuitable drying temperature range The temperature to which the substrateis subjected after having been lacquered and coated will vary with theparticular substrate. However, the particular temperature chosen, asnoted above, must be above the decomposition temperature of thedecomposable compound and it should be no more than 1000 F. below thesolidus temperature of the substrate. However, the decompositiontemperature should be at least 350 F.

The length of time necessary to cause diffusion and surface alloyingwill of course depend on the substrate, alloying metals and decomposablecompounds used, as well as the size of the part being surface alloyedand the degree of diffusion desired. Generally speaking, however, from 1minute to 4 hours will be sufficient.

As a further feature of the invention, it has been found that superiorproducts result when the alloying step is carried out in an atmosphereof dry gases such as dry hydrogen, cracked ammonia, endothermic andexothermic generated gas, and even dry steam, but not limited to these.Furthermore, it is especially advantageous when a reducing atmosphere isused, as the alloyed surface is more dense and more completely attachedto the metal substrate. Also, a very important feature of the inventionis the use of dry hydrogen as the reducing atmosphere, as such anatmosphere renders many compounds decomposable which would not decomposein air and also\ reduces the decomposition temperature of many of thedecomposable compounds While the mechanism by which the presentinvention is accomplished is not completely understood, and while notbeing bound by theory, the following theoretical explanation will aid inunderstanding the invention, and in reference to the accompaying drawingwherein;

The figure is a diagrammatic illustration of the product of theinvention.

In the figure, 1 is the substrate metal which may be any conventionalmetal or alloy such as, aluminum, st i less steel, nickel base alloys,cobalt base alloys, steel, copper, etc., 2 is the large atomic volumeelement of the decomposable compounds which have been decomposed. Thealloying elements are denoted by 3 and 4 represents the individual atomsor molecules of the alloying elements.

The diffusion zone (or zone of alloy) formed by the alloying materialsdiffusing toward the substrate and the substrate diffusing toward thealloying materials is denoted generally by 5. Hence, 5 is a solidsolution of the alloying metals and the metal substrate. It is to beclear ly understood that the lines of demarcations 6 and 7 between thealloying materials and the metal substrate are merely shown to explainthe invention and are not intended to suggest that such clear linesactually exist. Actually, the alloyed surface is a solid solution withno distinct lines of demarcation between the alloyed surface and thesubstrates similarly, the relative thickness and distinction between thesolid substrate 1 and the alloying metals 3 is not intended to be anactual depiction, but only to illustrate the invention of course theactual thickness of the alloyed coating may be as desired and iscontrolled in part by the thickness of the coating prior to diffusionand the extent diffusion is allowed to take place. The coating could beas little as micron or as great as inch.

When the substrate 1 has been coated with the lacquer containing thedecomposable compound, dried, coated with the solution of alloyingmaterials and dried, it is ready for subjecting to the elevated alloyingtemperature. As the temperature is raised to or past the decompositiontemperature of the decomposable compounds, the compounds decompose andthe elements become nascent. The nascent elements are easily combinedwith an available atoms. Hence, for example, if the large element of thedecomposable compound is barium, the barium atoms will readily combineand firmly attach to or slightly in the metal substrate of, for example,steel. When a dry hydrogen atmosphere is used, the other element orelements of the decomposable compound, such as a sulfate radical whenbarium sulfate is used will combine with the hydrogen to form H 50 andreadily vaporize off whereby the unneeded sulfate radicals are removed.

The large nascent atoms of the decomposable compound will attach to themetal subbstrate and will not substantially diffuse into the substrate.For example, if barium sulfate is the decomposable compound and iron isthe substrate metal, the nascent barium atoms will attach to the ironatoms on decomposition of the barium sulfate, but will not substantiallydiffuse into the iron since the atomic volume of barium is 39 and ironis only 7.1. Therefore, the larger barium atoms will be substantiallyprevented from diffusing into the substrate of smaller iron atoms whilethe small iron atoms of the substrate, to a much larger extent, candiffuse into and through the barrier layer of barium atoms. Similarly,the smaller alloying atoms can diffuse through the barrier layer ofbarium atoms. It will be appreciated from the above, that only a oneatom thick layer of the decomposable compound is necessary, but inpractice this is almost impossible to achieve, and from a practicalstandpoint, the thickness of the layer may be as great as A inch orgreater.

As a result of the above explained action, the objects of the inventionare accomplished. Hence, when a metal article such as a jet engine partis surface alloyed accordmg to the present invention, the barrier layersubstantially reduces the amount of further diffusion of the alloylngmetals into the substrate at the temperatures encountered during use(which temperature of necessity must be safely below the solidustemperature of the substrate or alloyed surface).

From the above discussion, it will be apparent that the atoms making upthe barrier layer must have a substantially larger atomic volume thaneither the substrate or the alloying metals. While some benefits can beobtained with barrier atoms only slightly larger than the alloying atomsand the substrate atoms, the barrier layer should preferably have anatomic volume of at least 1.25 times that of the alloying and substratemetal atoms and desirably more than 1.75 times larger atomic volume.

For practical application even better results are obtainable, since inmany applications the barrier layer atomic volumes can be 2 times orgreater than that of the alloying and substrate atomic volumes. Table 1below, which shows a number of common metals and alloying materials,illustrates the many possible combinations. Notice most of the commonmetals have an atomic volume of about or less, while only a few havegreater atomic volumes. Also notice that the last nine elements haveatomic volumes of at least 20, most of which are from group I-A and II-Aof the periodic table. These elements would of course do very well asbarrier layer atoms for the common metals and alloying metals.

TABLE 1 Elements: Atomic volume, w./.d Iron 7.1

Cobalt 6.7 Beryllium 5.0 Magnesium 14.0 Yttrium 19.8 Lanthanum 22.5

Titanium 10.6 Zirconium 14.1 Hafnium 13.6

Vanadium 8.35 Chromium 7.23

Molybdenum 9.4 Tungsten 9.53 Manganese 7.39 Rhodium '8.3 Iridium 8.54Nickel 6.6 Palladium 8.9 Platinum 9.1 Copper 7.1 Lead 18.3 Silver 10.3

Gold 10.2

Zinc 9.2 Cadmium 13.1 Aluminum 10.0 Silicon 12.1 Na 23.7 K 45.3 Rb 55.9Cs 70 Ca 29.9 Sr 33.7 Ba 39 Te 20.5 I 25.7

A very important feature of this invention is the great latitude itprovides in choosing particular metal substrates, barrier layers andalloying materials, and, accordingly, is applicable to a wide variety ofparticular uses.

The following examples will serve to illustrate the invention, but theinvention is limited only by the annexed claims.

EXAMPLE 1 Two identical parts used in the combustion chamber of a jetengine were cleaned by brushing. The parts were made of Hastelloy X. Alacquer was prepared by dissolving 15 parts barium chromate in 50 partsof a 5% solution of pyroxylin in Cellosolve. This lacquer was painted onthe parts and dried at 140 F. for 10 minutes. An aqueous solution wasprepared having the following composition by weight:

Percent Cobalt chloride Ammonium silicate 5 Water The aqueous solutionwas coated on the parts and dried at 240 F. for 10 minutes. The partswere then subjected to a temperature of 2100 F. for 3 minutes in anatmosphere of dry hydrogen. After cooling the parts slowly to roomtemperature, the surface alloyed parts were vapor blasted.

One of the parts was stored at room temperature while the other part wassubjected to 2000 F. for 24 hours. The two parts were then identicallysectioned and the part held at the elevated temperature was comparedwith the part held at room temperature. These was no evidence of furtherdiffusion in the part held at elevated temperature.

EXAMPLE 2 F. Barium chloride, 10 w./w. percent 2000 Calcium chloride, 10w./w. percent 1000 Potassium chloride, 10 w./w. percent 2000 The resultsin each of the above tests showed that no further diffusion takes placein the part held at the elevated temperature.

EXAMPLE 3 Ten metals alloying solutions were prepared by dissolvingvarious proportions of the following compounds in a 4% aqueous solutionof ethyl silicate:

Chromium chloride Nickel sulfate Iron oxide Cobalt chloride Aluminumchloride Columbiu'm chloride Molybdenum disulfide Manganous sulfateZirconium oxide Hafnium oxide Tungsten bromide Yttrium oxide Ammoniumsilicate The solutions were saturated and had the following compositionsof the metal ions by weight:

COMPOSITION, PART BY WEIGHT Chro- Alumimium IlLlIH Tita- Cobalt niumColumbium Molyb- Mangadenum Zireonium Yttri- Tungsten Silicon 410stainless steel 303 stainless steel Hastelloy X 1010 steel The partswere cleaned and lacquered with a solution of pyroxylin in Cellosolveand containing 35 parts/hundred by weight of barium chromate as thedecomposable compound. After drying at 180 F. for minutes, one part madeof each metal was coated with each of the metals alloying solution,dried at 240 F. for 10 minutes and subjected to a dry hydrogenatmosphere at 2000 F. for 30 minutes. Each part was sectioned and thealloy coating analyzed for the percent of each metal in the coating. Thecoatings were determined to have the same compositions as the metalalloying solutions noted in the table above.

EXAMPLE 4 The procedure of Example 3 was repeated except the jet engineparts were made of Rene 41 and Income] 702 using solutions 2, 4 and 10but without the silicon in solutions 2 and 4. Upon inspection of thefinished parts, a well-adhered alloyed coating was apparent.

EXAMPLE 5 The procedure of Example 1 was repeated except aluminum wasused as the metal of the engine parts and palladium chloride in asolution was coated thereon. The diffusion temperature was 1000 F. Thesame results as in Example 1 was found, i.g. no evidence of furtherdiffusion.

Having described the invention, it is readily apparent to one skilled inthe art that a number of modifications thereof may be made withoutdeparting from the spirit of the invention and such modifications are tobe contemplated as embraced within the scope of the following claims.

What is claimed is: 1. A process for alloying a metal substrate with analloying metal comprising (1) applying a first coating on said metalsubstrate of at least one decomposable inorganic compound in anon-carbonizing-film-forming material, said decomposable compoundcontaining at least one element selected from the group consisting ofNa, K, Rb, Cs, Mg, Ca, Ba, Sr, Zr, Hf, Te, Pb, and I and having adecomposable temperature of at least 350 F. and

at least 50 below the solidus temperature of the 1 metal substrate saidelement having an atomic volume at least 1.25 times the atomic volume ofelements of the said metal substrate and alloying metal,

(2) drying said coating on said substrate,

(3) applying a second coating of an aqueous solution of at least onereduceable salt or oxide of an alloying metal,

(4) drying said second coating,

(5) subjecting the so-coated substrate in a dry, gaseous atmosphere toan elevated temperature sufficient to decompose said decomposablecompound and thereby forming a barrier layer of said at least one largeelement of the said decomposable compound and to reduce said reduceablesalt or oxide to an alloying metal capable of alloying with said metalsubstrate thereby forming a surface alloy coating, and whereby saidlarge element of said barrier layer is disposed on or partially in thesurface of said metal substrate and said alloying metal is disposed onand through said barrier layer.

2. A process according to claim 1 wherein the atomic volume of the saiddecomposable compound is at least 1.75 times the atomic volume of theelements of the said metal substrate and the said alloying material.

3. A process according to claim 1 wherein the said gaseous atmosphere isdry hydrogen.

4. The process according to claim 1 wherein the said coating of alloyingmetal is applied in an aqueous solution containing a member selectedfrom the group consisting of organic borates and silicates, andinorganic borates and silicates.

5. The process of claim 1 wherein the said elevated temperature isWithin the range of 50 F. to 1100 F. below the solidus temperature ofthe metal substrate and above the decomposition temperature of thedecomposable compound.

6. The process of claim 1 wherein the decomposiable compound containssodium.

7. The process of claim 1 wherein the decomposable compound containspotassium.

8. The process of claim 1 wherein the decomposable compound containsrubidium.

9. The process of claim 1 wherein the decomposable compound containscesium.

10. The process of claim 1 wherein the decomposable compound containscalcium.

11. The process of claim 1 wherein the decomposable compound containsstrontium.

12. The process of claim 1 wherein the decomposable compound containsbarium.

13. The process of claim 1 wherein the decomposable compound containszirconium.

14. The process of claim 1 wherein the decomposable compound containshafnium.

15. The process of claim 1 wherein the decomposable compound containstellurium.

16. The process of claim 1 wherein the decomposable compound containsiodine.

17. The process of claim 1 wherein the decomposable compound containslead.

18. The process of claim 1 wherein the decomposable compound is selectedfrom barium chloride, barium chro mate, potassium chloride and potasiumchromate.

19. A process for alloying a metal substrate comprising 1) applying afirst coating on said metal substrate of at least one decomposableinorganic salt in a noncarbonizing film-forming resin, said decomposablesalt containing at least one element selected from the group consistingof Na, K, Rb, Cs, Ca, Ba, Sr, Zr, Hf, Te, and I and having adecomposable temperature of at least 350 F. and at least 50 below thesolidus temperature of the metal substrate,

(2) drying said coating on said substrate,

(3) applying a second coating of an alloying material comprising anaqueous solution of at least one reduceable inorganic salt of analloying metal and a member selected from the group consisting oforganic borates and silicates, and inorganic borates and silicates,

(4) drying said second coating,

(5) subjecting the so-coated substrate in a dry gaseous atmosphere to anelevated temperature sufiicient to decompose said decomposable saltthereby forming a barrier layer of at least one of said elements and toreduce said rcduceable salt to a metal capable of alloying with saidmetal substrate thereby forming a surface alloy coating, whereby saidelement of said barrier layer is disposed on or partially in the surfaceof said metal substrate and said alloy coating is disposed on andthrough said barrier layer, the atomic volume of said element being atleast 1.25 times the atomic volumes of the elements of said metal substrate and alloying metal.

20. The product produced by the process of claim 1.

(References on following page) References Cited UNITED STATES PATENTSOTHER REFERENCES Dantsizm 368,502 published May 11, 1943.

5 ALFRED L- LEAVITI, Primary Examiner Homer et al. C. K. WEIFFENBACH,Assistant Examiner McFarland.

Ross et al. US. Cl. X.R.

Kramer 117-127 X A.P.C. Application of Eduard Nachtigall, Ser. No.

